In the early days of autofocus SLRs, the motor that spun the lens's focus ring was usually in the lens itself. Eventually, most manufacturers shifted the location of the motor to the camera body. The exception was Canon, whose autofocus lenses each have an internal motor for autofocusing. With the motor in the body, a better body can bring new features to an existing (and oftentimes costly) set of lenses. On the other hand, with the motor in the lens, the motor can be customized to accommodate the lens's particular size and weight, which can vary greatly among lenses of different focal lengths.
High-end SLR cameras have a continuous autofocus feature that allows the user to take sharp pictures of a subject moving toward or away from the camera. When the shutter button is pressed halfway down, the autofocus system focuses on the moving object and begins tracking it. When the button is pushed all the way, the lens continues tracking the object right up to the moment the shutter actually opens, which can be a quarter of a second after the button is fully depressed.
According to their manufacturers, the best modern autofocus systems can work in light levels down to an exposure value (EV) of -1, which corresponds to an amount of illumination that would require an exposure of four seconds and an aperture of 1.4 for a film speed of 100. Early systems could not work in light levels below an EV of about+4, which corresponds to an exposure of 1/8 second at the same aperture and film speed. At very low light levels, the better systems focus on a pattern of red light, projected briefly from the flash unit onto the subject.
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